Spreading along the Lao and Vietnamese borders north of Ban Lung, Virachey is one of the largest protected forests in Cambodia — 3,300 sq km of dense evergreen jungle, river valleys and the mountainous Veal Thom grasslands. Multi-day treks with local guides (organised through the park headquarters in Ban Lung) are the only way to see it; expect hammock-camping, jungle leeches, and the genuine possibility of being the first foreigner the local communities have seen in months.
Treks run 3–8 days and cost USD 150–400 per person depending on the route. The wet season is more dramatic but the dry season (December–April) is much easier walking. Bring everything you might need from Phnom Penh — Ban Lung's shops are basic.